In the last decades, the search for sustainable and renewable feedstocks has increased considerably as crude oil based products are expected to become more limited in the future. One of these promising feedstocks is lactic acid, which is made by fermentation of glucose, which is available, for example, from corn or wheat. Due to its widespread availability, lactic acid has become more and more attractive for industries which require larger amounts of this material, such as coatings and inks industries. U.S. Pat. No. 7,588,632 describes water-based flexographic ink with polyactic acid dispersions.
Among printing inks, energy curable printing inks typically consist of a binder, a pigment, an energy curable acrylated monomer, and additives. Unlike conventional inks, which are often based on colophony derivatives and vegetable oils, energy curable inks are mainly made of oil-based chemicals, such as acrylic acid esters, and usually contain a low amount of renewable materials, and there is a desire to increase the amount of renewable material in energy curable inks by using resins based on sustainable materials.
What makes resins based on lactic acid especially suitable for energy curable food packaging inks are their outstanding organoleptic properties. Lactic acid itself is a food additive, and polymeric derivatives of lactic acid are used as food containers, fibers for clothing, or in medical human implants. Thus, such lactic acid materials are safe, and small amounts of residual lactic acid impurities would present a very low health risk in a lactic acid polyester resin, because residual lactic acid monomer is itself an edible food additive.
Attempts to use insoluble higher molecular weight poly(lactic acid) in flexographic inks in the form of an aqueous dispersion are described, for example, in U.S. Pat. No. 7,588,632. However, for making a suitable grinding varnish for an energy curable lithographic ink with good fineness of grind, the lactic acid polyester resin must have a good solubility in acrylic monomers.
On the other hand, typical high molecular weight poly(lactic acid) is insoluble in most common organic solvents, and especially insoluble in acrylic monomers for energy curable lithographic inks, such as trimethylolpropane triacrylate, and ethoxylated derivatives such as ethoxylated pentaerythritol tetraacrylate. Moreover, lactic acid oligomers or degraded poly(lactic acid), available from commercial sources, often exhibit crystallinity and poor solubility in acrylic monomers.
Thus, there is a constant search for alternative sustainable materials such as lactic acid derivatives in compositions and inks, which can replace existing oil-based products without adversely affecting the performance properties.